1. Field of the Invention
This invention relates to a method of treating a Ti(titanium)-Ni(nickel) shape memory alloy to improve various characteristic properties thereof.
2. Description of the Prior Art
A shape memory alloy formed of a Ti-Ni alloy with about 1:1 composition ratio of Ti to Ni is a substance called an intermetallic compound which has metallic bonding but exhibits properties similar to those of covalent bonding. In this Ti-Ni shape memory alloy, there occur various phenomena such as martensitic transformation, precipitation and oxidation in a very complicated way, whereby it is difficult to detect each of its phases exactly. In consequence, different views on the transformations of the alloy have been expressed by various researchers, and no satisfactory heat treatment method for the Ti-Ni shape memory alloy has been established yet.
In general, a Ti-Ni shape memory alloy commercially available, which is normally wire-shaped, has been obtained as follows. An ingot of the Ti-Ni shape memory alloy is hot worked into a rough shape at a high temperature of 800.degree. to 1000.degree. C., and thereafter, cold working and stress relief annealing are alternately repeated, to thereby make the ingot gradually approach a predetermined shape. This is because the Ti-Ni shape memory alloy exhibits high work-hardening, whereby it is difficult to carry out cold working such as wire drawing. The Ti-Ni shape memory alloy worked into a wire, etc. through the above-described processes is supplied to users in a hardened state after the final cold working.
The following three types of methods have heretofore been known as the treatment for giving a required shape to the shape memory alloy material in the hardened state as described above.
The first method is the one wherein the material supplied in a work-hardened state is further cold worked into a required shape, and thereafter, held at a temperature of 400.degree. to 500.degree. C. for about 15 minutes to 1 hour with its shape being restrained.
The second method is the one wherein the material is held at a temperature of 800 .degree. C. or above for a predetermined period of time, rapidly cooled so that the structure thereof is normalized, thereafter, cold worked into a predetermined shape, and held at a temperature of 200.degree. to 300.degree. C. with its shape being restrained.
The third method is the one wherein the material is heated at a temperature close to 1000.degree. C. to become perfectly a solid solution, thereafter, quenched, and aging-processed at a temperature of about 400.degree. C. This method is utilized only for a Ti-Ni alloy with a Ni concentration of more than 50.5%.
In general, the shape memory alloy given a certain configuration (shape or length) by one of the above-described methods has heretofore been used so that deformation thereof may not exceed 2% (if number of repetitions are particularly large, the deformation should be specifically 0.5% or less ) and the highest heated temperature may be rather low, i.e. its A.sub.f point +60 degrees or less. In practice, in order to extract a larger deformation through smaller strain, in most cases, the shape memory alloy has been used in a coil spring-like shape and heated indirectly through fluid such as air or water.
When deformation-shape restoration process is applied repeatedly under proper conditions to the alloy which is given a certain configuration by one of the above-described methods, as the number of repetitions is increased, the alloy comes to be stable in shape and smooth in movement. In general, this phenomenon is called learning effect, and sometimes utilized positively as a process for training the shape memory alloy. However, this training requires a considerable number of repetitions, and moreover, as the number of repetitions increases, plastic strain is generated and accumulated gradually in the alloy, thus presenting such problems that the range of motion of the alloy is reduced, and fatigue fracture is brought about.
Further, when the Ti-Ni shape memory by one of the above-described methods is heated by electric current passed therethrough, the wire tends to be overheated to lose the memory of the given configuration and to be broken. For example, when the Ti-Ni alloy wire is used in an actuator of type where the alloy is given tension and heated by electric current passed therethrough, the alloy gradually loses the memory of the given length, whereby length adjustment of the wire is frequently needed.
Now, in general, specific characteristic properties of crystalline materials are based on the phenomena in crystal grains of the materials. Accordingly, in many cases, these specific characteristic properties should naturally be most remarkably recognized when the materials are of single crystal. For this reason, when the excellent properties or functions of some material are to be utilized, in general, the best results can be obtained at the time the material is of single crystal. However, in practice it is extremely difficult to industrially produce the material of single crystal, and the production, even when achieved, should be very expensive. In consequence, most of materials, which have heretofore been actually used, are polycrystalline materials which can be easily produced, and in general, orientations of the respective crystals thereof have been random. The Ti-Ni shape memory alloy is no exception on this matter.
With polycrystalline materials which have the above-described random crystal orientations, including the Ti-Ni shape memory alloy, "the excellent characteristic properties or functions" cannot be extracted to the utmost as with single-crystal materials from the following reasons.
(A) The above-described "excellent characteristic properties or functions" are not exhibited at grain boundaries or thereabout.
(B) In general, "the excellent characteristic properties or functions" do not equally appear in all of the directions, but appear to the utmost in a specific direction depending on the orientation of the respective crystal. However, since the crystal orientations are random, "the excellent characteristic properties or functions" cannot be extracted to the utmost in any directions, in view of the material as a whole.